A camera's autofocus (AF) system automatically adjusts the camera lens to obtain focus on a subject. In general, AF systems use contrast sensors within the camera (passive AF) or by emitting a signal to illuminate or estimate the distance to a subject (active AF). Passive AF may employ either contrast detection or phase detection methods, but both rely on contrast for achieving autofocus.
Some digital cameras utilize autofocus sensors that are different from the camera's image sensor. This may, for example, occur in single-function digital cameras. In small multifunction devices such as mobile phones, tablet computers, personal digital assistants and portable music/video players, however, separate image and autofocus sensors are often not used. In devices such as these, autofocus operations can involve adjusting the position of the camera lens (or lens assembly) at a specified number of locations; and evaluating the focus (e.g., contrast) between corresponding points in successive images (maximum contrast is assumed to correspond to maximum sharpness or “best” focus).
Multifunction devices typically evaluate contrast/focus at a fixed number of points in an image and, when the lens is moved to obtain successive images, a fixed number of lens positions or points of interest (POI). As such, these devices can provide autofocus operations at a fixed rate and with a fixed resolution.
US 2013/0076967 describes an autofocus operation for a digital image capture device, using knowledge of the device's orientation. The method includes determining the orientation of the device from an orientation sensor where one illustrative orientation sensor is an accelerometer. Based at least in part on the device's orientation, a range of motion may be determined for the autofocus operation. This range of motion refers to the distance through which a lens, or lens assembly, is moved during the autofocus operation.
However US 2013/0076967 assumes that the effects of device orientation are linear in nature and further assumes that the actuator characteristics of individual devices are consistent at different orientations. In fact this is not the case and actuator characteristics vary significantly between orientations and, in turn, pre-defined POIs are not sufficiently accurate to enable auto-focus algorithms to work correctly at non-horizontal orientations.
U.S. Pat. No. 8,577,216 discloses calibrating a lens actuator used to move a lens within a search range during an auto-focus process in an image capture device. The image capture device adjusts reference positions for the search range based on lens positions selected for different focus conditions, the different focus conditions including a far focus condition and a near focus condition. The focus conditions are determined based on a detected environment in which the device is used. Detection of an indoor environment may indicate a likelihood of near object focus, while detection of an outdoor environment may indicate a likelihood of far object focus.